Antares Nuclear Inc.’s Mark-0—a sodium heat-pipe-cooled microreactor fueled by high-assay low-enriched uranium (HALEU) tri-structural isotropic (TRISO) fuel compacts—has achieved zero-power criticality at Idaho National Laboratory’s (INL’s) Reactor and Critical Experiment (RACE) facility, becoming the first advanced reactor to reach that milestone under the Department of Energy’s (DOE’s) Reactor Pilot Program.
The development, announced on June 4, also marks the 53rd reactor built at the INL site since 1951 and the first novel reactor design to achieve criticality at the laboratory in more than 50 years, according to INL Laboratory Director John Wagner.
The much-watched DOE Reactor Pilot Program, established under President Trump’s May 2025 Executive Order 14301, directs the DOE to accelerate reactor testing and to target at least three advanced-reactor criticalities by July 4, 2026.
“Criticality is the condition at which a nuclear fission chain reaction becomes self-sustaining,” Wagner explained in a LinkedIn post. “What Antares achieved is specifically zero-power criticality—the chain reaction was sustained at essentially no measurable energy output. This is not electricity generation. It is not full-power operation. It is proof that the system works: the scientific and engineering validation that every subsequent step depends on,” he wrote. “That distinction matters for context. It should not diminish what happened.”
From Startup to Criticality in Two Years
The zero-power criticality milestone marks a significant step for Torrance, California–headquartered Antares, which was founded in 2023 and has raised more than $140 million in private capital, including a $96 million Series B round that closed in December 2025. The company announced Jan. 26 that DOE had approved its Preliminary Documented Safety Analysis for Mark-0, calling the approval a key step toward fabrication, assembly, installation, and operation under the Reactor Pilot Program.
Antares began machining the Mark-0 graphite core on Jan. 12 at its Antares Prime facility, and fuel fabrication for its first reactors has been underway through BWX Technologies since October 2025 using HALEU secured through a DOE allocation. Antares says it holds agreements with the U.S. Air Force, Space Force, NASA, and the Defense Innovation Unit, and is advancing toward initial deployments for defense and space customers in 2028.
“Now that Mark-0 is critical, the real work is just beginning,” said Antares CEO Jordan Bramble in a LinkedIn post on June 4. “I want to reiterate how this fits into our larger roadmap to mature our technology to its commercial potential. This should be obvious, but the goal of a reactor is to sell electricity to customers.”
Following reactor physics experiments, Antares will execute “the next phase of our roadmap—sustained electricity production,” Bramble said. Antares is “able to move fast towards this milestone because we’ve already completed over 6 months of full-power thermal testing in an electrical prototype. We will perform version 2.0 of this in 2026. This is an easier, more iterative way to test, because there is no regulatory process, and you can disassemble to examine material effects.”
He added: “All of our iterative testing sets us up to produce electricity for 6+ months. Hundreds of days, not hundreds of hours. We’re able to test for longer and faster because we’ve designed our reactor around a proven, fully qualified fuel spec developed under Project Pele.”
A Sodium Heat-Pipe-Cooled Microreactor
The Mark-0 is a small, high-temperature, sodium heat-pipe reactor configured specifically for zero-power criticality testing, according to a DOE Idaho Operations Office categorical exclusion determination. Unlike a power-producing prototype, this version is “not equipped with power conversion or heat removal systems” and is designed to serve as “a validation platform for advanced nuclear reactor technologies.”
The reactor was installed below grade level inside a pit on the east side of the high bay of INL’s Sodium Components Maintenance Shop, Building MFC-793, at the Materials and Fuels Complex, a steel-framed facility which was chosen for its existing infrastructure, bridge crane capability, and ventilation system, which required structural modifications under INL document INL-25-027 to meet nuclear design criteria.
Bramble has said Antares designed the company from its inception around both terrestrial microreactor and space nuclear applications, suggesting that heat-pipe cooling is “one of the reasons” the architecture can serve both markets. Rather than circulating coolant through an active pump loop, the Mark-0’s design passively transfers heat through sealed, sodium-filled pipes. Sodium vaporizes near the fuel, migrates to a cooler end, condenses, and returns. That passive cooling is designed to eliminate the active-component failure modes that drove much of the safety analysis burden in earlier reactor designs, and the same architecture applies directly to space nuclear power systems, which cannot rely on active cooling infrastructure.
Fuel Chain: BWXT and Project Pele
The Mark-0 operates on HALEU—uranium enriched to less than 20% U-235—in TRISO fuel compacts, loaded at less than 120 kilograms total for the operational life of the reactor. TRISO particles coat uranium kernels in successive layers of carbon and silicon carbide, which contain fission products under high temperature and irradiation. Most advanced non-light-water designs require HALEU because their neutron physics demand higher enrichment than the roughly 4% U-235 used in conventional light-water reactor fuel.
However, no U.S. commercial enricher currently produces HALEU at commercial scale yet. While Centrus Energy’s 16-machine demonstration cascade in Piketon, Ohio, has produced just over 920 kilograms under a DOE contract—enough for early demonstration work—DOE and the National Nuclear Security Administration (NNSA) covered the Mark-0’s feedstock requirement by providing government-held scrap material, which BWX Technologies processed at its Specialty Fuels Fabrication facility in Lynchburg, Virginia, and fabricated into finished TRISO compacts.
For longer-term supply, Antares signed what Urenco described as “the world’s first multi-year” commercial HALEU supply contract in May 2026, under which Urenco will supply enrichment services from its Advanced Fuels Facility at Capenhurst in the UK. As POWER earlier this week reported, Urenco’s Advanced Fuels Facility is planned to come online in 2031 at an initial output of up to 27 metric tons per year—enough to supply up to 30 advanced reactors.
Antares, notably, modeled its fuel on TRISO compacts BWXT developed for Project Pele—the U.S. Army’s Strategic Capabilities Office program to build a 1.5-MW transportable microreactor. BWX Technologies said that TRISO fuel specification was developed within DOE’s Advanced Gas Reactor program over several decades and, paired with BWXT’s “decades of TRISO development” in Lynchburg, helped accelerate Antares’ path to criticality.
“BWXT’s TRISO fuel supported our path to criticality,” Bramble confirmed in a press release. “Building on a proven fuel specification developed through Project Pele let our team focus on what we had to prove ourselves: our control system and reactor physics. We’re grateful for a partnership that continues as we move from neutrons to electrons.”
BWX Technologies President and CEO Rex D. Geveden said the milestone underscores the company’s role in advanced fuel fabrication. “Our skilled workforce, advanced manufacturing technologies and nuclear-qualified supply chain are driving a new generation of reactor demonstrations across the country,” he said. Joe Miller, BWXT’s president for Government Operations, added that Antares is “moving quickly to progress from concept to criticality,” and BWX Technologies said it will continue supporting Antares with ongoing TRISO fuel manufacturing as the program advances.
—Sonal Patel is a POWER senior editor (@sonalcpatel, @POWERmagazine).
Editor’s note: This story is developing. POWER will update it as additional information becomes available.
